Cancer is America's second leading cause of death. Many approved cancer drugs, such as bortezomib (Velcade), are cytotoxic agents that kill normal cells as well as tumor cells. Therapeutic benefit depends on tumor cells being more sensitive than normal cells, thereby allowing clinical responses to be achieved at relatively safe drug doses;however, damage to normal tissues is unavoidable and often limits treatment. Due to its remarkable efficacy in multiple myeloma (MM), bortezomib has been tested in solid cancers;but it has generally failed to produce clinical responses. Bortezomib inhibits an intracellular protein complex called the proteasome. Because the proteasome regulates cellular physiology in both normal and tumor cells, bortezomib causes many dose-dependent side effects of which sensory and motor dysfunction called peripheral neuropathy (PN) and deficiency in platelets (thrombocytopenia) are the most severe. MM patients can be treated safely, because MM cells can be killed by bortezomib at doses that are tolerated, albeit not without frequent incidence of PN. In contrast, tumor cells of solid cancers are more resistant, and PN and thrombocytopenia prevent administration of an effective dose of bortezomib with acceptable safety. ARI-3996 is a pro-drug version of a bortezomib-like cytotoxic agent designed to more selectively target the proteasome in solid tumors. ARI-3996 is relatively non-toxic to all cells and cannot kill tumor cells until it is activated by the enzyme, fibroblast activation protein (FAP). Because FAP is produced in epithelial tumors but not usually in healthy tissues, ARI-3996 should not be activated in nervous tissue or in bone marrow where platelets are generated. Therefore, ARI-3996 should kill FAP-producing tumors with less severe PN and thrombocytopenia than that associated with bortezomib. If successful, ARI-3996 might be combined with chemotherapy to improve the current standards of care. FAP has been shown to specifically activate ARI-3996 and unleash the bortezomib-like proteasome inhibitor to kill tumor cells in tissue culture. It is now proposed to demonstrate that ARI-3996 can kill tumors in an animal model of epithelial cancer with less systemic toxicity than bortezomib. A mouse model will be selected in which the tumor produces FAP in amounts equivalent to those in human epithelial tumors. The dose-response of ARI-3996 will be determined for reduced tumor growth, death of tumor cells, and reduced blood supply to the tumor. Mice will be examined for toxic side effects in comparison to bortezomib. For ARI-3996 to be a viable drug candidate, it will be essential to demonstrate that treatment with ARI-3996 produces significant antitumor effects with at least 10-fold less systemic toxicity than bortezomib. PUBLIC HEALTH RELEVANCE: Bortezomib (Velcade) is an effective treatment for multiple myeloma, but its mechanism of action results in dose-limiting toxicities (DLTs) of peripheral neuropathy and loss of platelets, which prevent treatment of common solid cancers. The pro-drug, ARI-3996, is designed to remain inactive in healthy organs and to be activated to unleash a cytotoxic bortezomib-like warhead in tumors by the tumor-associated enzyme called fibroblast activation protein (FAP), thereby reducing the toxic side effects that prevent safe treatment of solid tumors with bortezomib. If ARI-3996 is shown to have a bortezomib-equivalent antitumor effect in a mouse model of FAP-producing epithelial cancer with at least a 10-fold reduction in toxicity compared to bortezomib, Phase II funding for IND-enabling studies will be applied for.